Piezoelectric micromachined ultrasonic transducer with a patterned membrane structure

ABSTRACT

A piezoelectric micromachined ultrasonic transducer (PMUT) device includes a substrate having an opening therethrough and a membrane attached to the substrate over the opening. An actuating structure layer on a surface of the membrane includes a piezoelectric layer sandwiched between the membrane and an upper electrode layer. The actuating structure layer is patterned to selectively remove portions of the actuating structure from portions of the membrane to form a central portion proximate a center of the open cavity and three or more rib portions projecting radially outward from the central portion.

CLAIM OF PRIORITY

This application is a continuation of International Patent Applicationnumber PCT/US2017/062059 filed Nov. 16, 2017, the entire contents ofwhich are incorporated herein by reference.

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to micromachined ultrasonictransducers (MUTs) and more particularly to a design for a piezoelectricmicromachined ultrasonic transducer (PMUT) device and a method tofabricate such a device.

BACKGROUND OF THE DISCLOSURE

Micromachined ultrasonic transducers (MUTs) have been subject toextensive research for the last two decades. Piezoelectric micromachinedultrasonic transducers (pMUTs) are MUTs that use a piezoelectric layerfor electro-mechanical transduction. A typical pMUT is a multilayermembrane structure that is excited into flexural vibration usingpiezoelectric actuation. The membrane structure is often formed byetching through a silicon wafer to remove the material beneath themembrane, thereby allowing it to vibrate. This etch forms a hollow tubebeneath the backside of the membrane. Sound is emitted from the tubewhen the membrane vibrates, and the tube may be designed as an acousticresonator to improve acoustic performance of the pMUT. These devicestypically operate at the membrane's flexural resonance frequency, whichis defined by selecting the correct materials, membrane size, thicknessand/or in-plane stress. For multi-pMUT applications, good matching ofthe resonance frequencies of the individual pMUTs is required for properoperation. For this reason, it is important for pMUTs to be manufacturedwith closely-matched resonance frequencies. One important parametercausing frequency variation is the residual stress present in the layerscomposing the pMUT membrane, in particular in the piezoelectric layer. Atypical pMUT structure consists of a membrane that is attached to thesubstrate at its boundary, a condition that is described as a clampedboundary condition. The resonant frequency of a membrane with a clampedboundary condition is very sensitive to in-plane residual stress.Several designs have been suggested to reduce stress sensitivity, suchas released cantilevers and flexurally-suspended membranes, but they allshow poor acoustic performance and or poor manufacturability (e.g.inefficient resonance modes, cracks created by stress concentrations,poor micro-fabrication yield)

In US 2012/0250909 Grosh describes an acoustic transducer wherein amembrane transducer is released from the substrate by separating themembrane into several identical tapered cantilevers, reducing the effectof stress on the mechanical behavior of the membrane. Grosh's approachworks well for a non-resonant device, such as a conventionalpiezoelectric microphone. However, for a pMUT operated at resonance,small differences caused by fabrication variations can cause thecantilevers to have slightly different resonance frequencies, resultingin considerable negative impact on the acoustic performance of thedevice when operated at resonance. Specifically, when excited at asingle frequency, mismatched cantilevers will oscillate withsignificantly different phase and amplitude, creating phase andamplitude errors in the ultrasound signal.

International Patent Application Publication Number WO 2015/131083describes patterning the actuation structure of a pMUT with an electrodeproximate the membrane edge with ribs radiating inward. Such patterning(sometimes referred to as a “ring” electrode) can reduce residual stressin the membrane. In some sense, however, an actuating structure with a“central” electrode has much better performance than the “ring”electrode. In particular, the electromechanical coupling is higher forthe central electrode structure than for the ring electrode structure.Unfortunately pMUTs with a central electrode generally have morefrequency variation because they have piezo material over the entiresurface, and the piezo material's stress varies significantly across thewafer.

Accordingly, what is needed is a pMUT design with good acousticperformance that resonates at a single stable resonance mode and withlow sensitivity to stress.

SUMMARY

According to aspects of this disclosure a piezoelectric micromachinedultrasonic transducer (PMUT) device may include a substrate having anopen cavity, a membrane attached to the substrate, and an actuatingstructure on a surface of the membrane. The actuating structure layerincludes a piezoelectric layer sandwiched between the membrane and anupper electrode layer. The actuating structure is patterned so thatportions of the actuating structure are selectively removed fromportions of the membrane to form an actuating structure having a centralportion proximate a center of the open cavity and three or more ribportions projecting radially outward from the central portion.

In some implementations the membrane may be attached to the substrate atone or more anchor portions of the membrane proximate the perimeter ofthe open cavity. In some such implementations the ribs may extend fromthe central portion to, or beyond, the anchor points of the membrane.

In some implementations, there may be four or more ribs, six or moreribs, or eight or more ribs.

In some implementations, one or more of the three or more ribs may bepatterned such that it is mechanically coupled to the central portionbut electrically isolated from the central portion.

In some implementations, the actuating structure may be encapsulated bya passivation layer. In some such implementations the passivation layermay be patterned such that it is substantially removed from the portionsof the membrane not covered by the actuating structure layers.

In some implementations, the ribs may include one or more tapered ribportions. Such tapered rib portions may be wider proximate the perimeterof the membrane than at the central portion.

In some implementations, the membrane layer may be perforated with oneor more holes that pass through the actuating structure and the membranelayer.

In some implementations the actuating structure layer may include alower electrode layer sandwiched between the piezoelectric layer and themembrane.

In some implementations the perimeter of the open cavity may be circularin shape.

In some implementations the perimeter of the open cavity may be squarein shape.

In some implementations the perimeter of the open cavity may bepolygonal in shape.

In some implementations the three or more rib portions may include fouror more rib portions.

In some implementations the three or more rib portions may include sixor more rib portions.

In some implementations the three or more rib portions may include eightor more rib portions.

In some implementations the three or more rib portions may include oneor more tapered rib portions.

In some implementations the three or more rib portions may include oneor more tapered rib portions that are wider proximate the peripheralportion than at the central portion.

Some implementations of pMUT devices in accordance with aspects of thepresent disclosure may further include a passivation layer formed overthe actuating structure.

Some implementations of pMUT devices in accordance with aspects of thepresent disclosure may further include an opening formed through thecentral portion of the actuating structure and through an underlyingportion of the membrane to the open cavity.

In some implementations a ratio of a radius of the central portion ofthe actuating structure to a radius of the membrane layer may be between0.4 and 0.8.

In some implementations a ratio of a radius of the central portion to aradius of the membrane layer may be about 0.6.

Aspects of the present disclosure include a method for fabricating apiezoelectric micromachined ultrasonic transducer (PMUT) device. Themethod generally involves forming a membrane attached to a substrate;forming an actuating structure on a surface of the membrane, theactuating structure layer including a piezoelectric layer sandwichedbetween the membrane and an upper electrode layer; patterning theactuating structure to selectively remove portions of the actuatingstructure from portions of the membrane to form an actuating structurehaving a central portion proximate a center of the membrane and three ormore rib portions projecting radially outward from the central portion;and forming an opening through a portion of the substrate underlying themembrane and actuating structure.

BRIEF DESCRIPTION OF THE FIGURES

The present disclosure may be better understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 shows a cross section of a circular clamped transducer designknown from prior art.

FIG. 2 is a cross section of a pMUT taken along line 2-2 of FIG. 3according to aspects of the present disclosure.

FIG. 3 is a top view of a partially fabricated pMUT according to aspectsof the present disclosure.

FIG. 4A is a cross section of a rib portion of an actuating structurefor the pMUT shown in FIG. 2 and FIG. 3 taken along line A-A of FIG. 3 .

FIG. 4B is a cross section of a rib portion of an actuating structurefor the pMUT shown in FIG. 2 and FIG. 3 taken along line B-B of FIG. 3 .

FIG. 4C is a top plan view of a rib portion of an actuating structurefor a pMUT according to aspects of the present disclosure.

FIGS. 5A-5K are a sequence of cross-sectional schematic diagramsillustrating fabrication of a pMUT device in accordance with aspects ofthe present disclosure.

DETAILED DESCRIPTION

Although the description herein contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments, which maybecome obvious to those skilled in the art.

INTRODUCTION

To overcome issues of residual stress present in the piezoelectric layerthat affects the stiffness of the membrane of a pMUT device, aspects ofthe present disclosure include a patterned actuating structure havingselected portions removed from portions of an underlying membraneleaving an actuating structure with a central portion proximate a centerof the membrane and three or more rib portions projecting radially. Suchpatterning provides the advantages of having a central electrode whilereducing residual stress and associated frequency variation.

Piezoelectric Micromachined Ultrasonic Transducer (PMUT) Device

A piezoelectric micromachined ultrasonic transducer (PMUT) device withinthe context of the present disclosure may be more fully appreciated bycomparison to existing pMUT designs. By way of example, FIG. 1 depicts atypical conventional art pMUT device 100 formed from multiple thin-filmlayers deposited onto the substrate 101. The device is formed on asubstrate 101, e.g., made of silicon but alternative materials such asglass or polymer substrates may be used. A bottom layer 102 is formed ona surface of the substrate. A bottom electrode layer 104 is sandwichedbetween the bottom layer 102, and a piezoelectric layer 103. A topelectrode 106 is formed on the piezoelectric layer 103. An opening 107is formed through the substrate 101 from a backside thereof all the waythrough to the underside of the bottom layer 102.

The bottom layer 102 may be made of silicon, silicon dioxide, and/orsilicon nitride, among other materials. The piezoelectric layer 103 maybe composed of various piezoelectric materials including AIN and alloysof AIN such as Sc_(x)Al_(1-x)N, PZT (lead zirconate titanate) and alloysof PZT such as PLZT and PNZT, ZnO, KNN (K_(x)Na_(1-x)NbO₃) or PMN-PT(lead magnesium niobate—lead titanate). Polymer piezoelectric materialssuch as PVDF may be sometimes be used as piezoelectric layer 103.Various metals may be used for bottom electrode 104 and top electrode106 including Al, Au, Pt, Cu, and Mo. Where the bottom layer 102 is madeof a sufficiently electrically conductive material, such as dopedsilicon, the bottom electrode layer 104 may be omitted.

One of the problems with a pMUT design like that shown in FIG. 1 is thatresidual stress can be present in the piezoelectric layer that affectsthe stiffness of the membrane. In general, the membrane's stiffness isdetermined by both the residual stress (e.g. tension or compression) andthe membrane's flexural rigidity. The flexural rigidity is determined byparameters that can be tightly controlled during the manufacturingprocess, namely the membrane geometry (e.g. thickness and diameter) andmaterial properties (e.g. Young's modulus and Poission's ratio).However, residual stress, particularly in the piezoelectric layer andelectrode layers, is more difficult to control and variations in thisresidual stress from device-to-device and wafer-to-wafer candetrimentally affect the resonant frequency and overall acousticperformance of the pMUT device 100, leading to undesirable manufacturingvariations in the pMUT devices. To overcome these problems, aspects ofthis disclosure include a micromachined ultrasonic transducer (MUT), inparticular a piezoelectric micromachined ultrasonic transducer (pMUT)having a variable thickness structure that includes portions of thepiezoelectric layer and electrode layer(s). Portions of the piezo stack(i.e., the structure containing the piezoelectric layer andelectrode(s)) are removed from the membrane, forming rib-likestructures. These rib-like structures substantially maintain theflexural rigidity of the membrane, but substantially reduce thecontribution of the piezo stack's residual stress on the membranestiffness.

Variable Thickness Structure

FIG. 2 and FIGS. 4A-4B are cross-section illustrations of portions of apMUT device 200 in accordance with aspects of the present disclosure.FIG. 3 is a corresponding plan view of the device. The device 200 may beformed from multiple thin-film layers deposited onto a substrate 201.The substrate 201 may be silicon but alternative materials such as glassor polymer substrates may be used. An opening 207 may be formed thoughthe substrate 201. In the illustrated pMUT device 200, a sacrificiallayer 202 is formed on a surface of the substrate 201. Sacrificial layer202 may be made of various materials such as silicon dioxide, and thethickness of this layer is in the range of 0.1 microns to 4 microns.

An elastic layer 204 is formed on the sacrificial layer 202 and over theopening 207 in the substrate 201. The elastic layer 204 may be made ofsilicon, silicon dioxide, and/or silicon nitride, among other materials.By way of example, and not by way of limitation, the elastic layer 204may be made of polycrystalline silicon (polysilicon). The elastic layer204 may be anchored directly to the substrate 201 at one or more anchors213 formed through openings in the sacrificial layer 202. A thickness ofthe elastic layer 204 may range from, e.g., about 1 microns to about 20microns for transducers with center frequency from 40 kHz to 30 MHz, andmore specifically from 1 micron to 20 microns for transducers withcenter frequency from 40 kHz to 1 MHz. A small vent opening 205 may beformed through the membrane layer to facilitate equalization of pressurebetween the two sides of the membrane layer. The vent opening 205 isgenerally relatively small compared to the size of the opening 207. Byway of example, and not by way of limitation, the diameter of theopening 205 may be between 2 microns to 50 microns. It is generallydesirable to locate the vent opening in some place where there is notmuch stress in the membrane 204, which may not necessarily be true atthe center. Examples of such vent openings are described, e.g., in U.S.patent application Ser. No. 15/141,746 filed Apr. 28, 2016 (published asPatent Application Publication Number 20170021391), U.S. patentapplication Ser. No. 15/625,421 filed Jun. 16, 2017, and InternationalPatent Application Number PCT/US17/36613 filed Jun. 8, 2017, the entirecontents of all of which are incorporated herein by reference.

An actuating structure layer 206 is formed on a surface of the membranelayer 204. In the illustrated example, the actuating structure layer 206includes a lower electrode layer 208 sandwiched between the elasticlayer 204 and a piezoelectric layer 210, which is sandwiched between thelower electrode layer 208 and an upper electrode layer 212. The bottomand top electrode layers may be made of various metals such as Mo, Pt,or Al, and the thickness of these layers may range from 50 nm to 500 nmand more specifically from 100 nm to 300 nm. The piezoelectric layer 210is formed over the bottom electrode 208. The bottom electrode layer 208may also extend beyond the piezoelectric layer 210 and cover portions ofthe elastic layer 204. The piezoelectric layer 210 may be made ofvarious piezoelectric materials including aluminum nitride (AlN) and itsalloys such as ScAlN, lead zirconate titanate (PZT) and its alloys suchas PLZT and PNZT, zinc oxide (ZnO), K_(x)Na_(1-x)NbO₃ (KNN) or leadmagnesium niobate—lead titanate (PMN-PT). Polymer piezoelectricmaterials such as PVDF may be used as piezoelectric layer 210 as well.The thickness of piezoelectric layer 210 may range from about 250 nm toabout 3000 nm, more specifically from about 500 nm to about 1500 nm. Inthe region spanning between the anchors 213 where the elastic layer 204is released from the substrate 201, the combination of the elastic layer204 and the actuating structure 206 forms a flexing part of the device200. For convenience, this flexing combination is referred to herein asthe membrane.

The actuating structure layer 206 is patterned to form an actuatingstructure having a central portion 206C proximate a center of themembrane, rib portions 206R projecting radially and extending to theedge or beyond the edge of the membrane, and open portions 206′ that arefree of the piezoelectric layer 210 and electrode layers 208, 212. Eachof the open portions borders the central portion and at least two of therib portions. In some implementations a ratio of a diameter of centralportion 206C to a diameter of the membrane may be between 0.4 and 0.8,e.g., about 0.6. In some implementations the rib portions 206R mayinclude one or more tapered rib portions. By way of example, and not byway of limitation, such tapered rib portions may be wider proximate theedge of the membrane than at the central portion 206C.

In some implementations a protective passivation layer 214 may be formedover the actuating structure 206 to encapsulate and protect the lowerelectrode 208, upper electrode 212, and piezoelectric layer 210 fromcorrosion e.g. due to humidity. In some embodiments this passivationlayer may extend over otherwise exposed portions of the membrane 204 inthe open portions 206′. Electrical connections to the electrodes may bemade through contacts 216, 218, respectively, formed in openings in thepassivation layer 214.

FIGS. 4A-4B show the cross-sectional structure of one of the ribportions 206R depicted in FIG. 3 . To facilitate understanding, thepassivation layer 214 is not shown in FIG. 3 . A top view of a ribportion 206R is shown in FIG. 4C.

As described earlier, the membrane's stiffness has two components, thefirst being the flexural rigidity of the membrane layers (e.g., theelastic layer 204 and the actuating layers 206) and the second being thetensile or compressive stress in these layers. The presence of the ribportions 206R keeps the flexural rigidity of the membrane comparable toa continuous, unpatterned membrane. However, the absence of theactuating structure 206 (piezoelectric layer 210, top electrode 212 andbottom electrode 208) in the open portions of the membrane reduces thecontribution of the stress in these layers to the stiffness of themembrane.

FIG. 3 illustrates and example of a partially fabricated pMUT device 200showing the patterned lower electrode layer 208 formed on the membranelayer 204. This example illustrates the device 200 at an intermediatestage of fabrication after the upper electrode 212 has been formed andpatterned on the piezoelectric layer 210, after which the piezoelectriclayer 210 has been patterned and finally the exposed bottom electrodelayer 208 has been patterned, and the resulting actuating structure 206has been fully encapsulated by the passivation layer 214. The electrodelayers 208, 212 and piezoelectric layer 210 may be patterned to definecentral portions 208C, 212C proximate a center of the membrane, and ribportions 208R, 210R, 212R projecting radially outward from the centerportion 208C, 212C and extending to or beyond the anchors 213 thatsupport the membrane edges, e.g., as seen in FIG. 4B. At least one ribportion is electrically connected to the center portion and to a contact215 while the other rib portions may be electrically isolated from thecenter portion since these rib portions do not contribute the actuationor sensing of vibration in the membrane. Lower electrode portions 208C,208R are denoted by dashed lines in FIG. 3 . In the example depicted inFIG. 3 there are eight (8) open portions 206′ where the membrane 204 isnot covered by the electrode layers. However, aspects of the presentdisclosure are not limited to such implementations and generallyencompass implementations in which there are more than three openportions, e.g., four or more, six or more, eight or more. In theillustrated example, the membrane 204 is covered by the passivationlayer 214 in the open portions. However, aspects of the presentdisclosure are not limited to such implementations and includeimplementations in which there is no passivation layer over the membranein the open portions.

A pMUT device of the type depicted in FIGS. 2-4C may be fabricated inaccordance with aspects of the present disclosure, e.g., as illustratedin FIGS. 5A-5L. Specifically, as shown in FIG. 5A, fabrication may startwith a substrate 201 having the sacrificial layer 202 formed thereon.The sacrificial layer may be formed by any suitable deposition or growthtechnique for the relevant material. Deposition techniques includephysical vapor deposition (PVD) and chemical vapor deposition (CVD).Growth techniques include thermal oxidation (e.g., where the sacrificiallayer is an oxide of the substrate material). Next, the sacrificiallayer 202 may be patterned to define anchor point openings 211, as shownin FIG. 5B. Such patterning may be accomplished with a conventionallithography followed by a suitable etch to form openings through thepassivation layer 202 to the surface of the substrate 201.

Next, as shown in FIG. 5C, the elastic layer 204 may be formed over thesacrificial layer 202, e.g., in a CVD or epitaxial reactor. The presenceof the anchor point openings 211 facilitates attachment of the membranelayer 204 to the substrate 201 by anchors 213. The elastic layer 204 isthen polished, e.g. using chem-mechanical polishing (CMP), to achieve alow surface roughness in preparation for deposition of the piezoelectricactuation structure. The lower electrode layer 208 is then formed, e.g.,by a metal deposition technique such as PVD. In some implementations aseed layer 208S, e.g., of AlN, may be formed on the elastic layer 204and the metal of the lower electrode layer 208 may be formed on the seedlayer. The piezoelectric layer 210 is formed on top of the lowerelectric layer 208 using a deposition technique such as PVD or chemicalsolution deposition, also known as sol-gel deposition. The upperelectrode layer 212 is then formed over the piezoelectric layer 210,e.g., by a metal deposition technique such as PVD, and is then patternedto form a central area 212C and rib portions 212R.

The piezoelectric layer 210 is then patterned as shown in FIG. 5D, e.g.,using an etching process that stops on the lower electrode layer 208.The lower electrode layer 208 is then patterned as shown in FIG. 5Ethereby defining the central portion 208C. Following patterning of thelower electrode layer, the passivation layer 214 is deposited,encapsulating the upper electrode layer 212, the piezoelectric layer210, and the lower electrode layer 208 as shown in FIG. 5E. By way ofexample, and not by way of limitation, the passivation material 214 maybe aluminum nitride (AlN), silicon nitride (Si₃N₄), silicon carbide(SiC) or another dielectric material. Alternatively, different materialsmay be used. In some embodiments, the passivation layer 214 may besubsequently removed from the open portions of the elastic layer 204 notcovered by the upper electrode layer 212, the piezoelectric layer 210,and the lower electrode layer 208. As shown in FIG. 5G, Openings O1, O2may be formed through the passivation layer 214 to allow for electricalcontacts to the upper electrode 212 and lower electrode 208. Anadditional opening O3 may be formed though the passivation layer todefine a vent opening.

Conductive contacts 216, 218 may be formed to the lower electrode 208and upper electrode 212, as shown in FIG. 5H. The contacts 216, 218 maybe made of a corrosion-resistant material such as gold. A vent opening205 may formed through the membrane layer 204 and any overlying layer,such as the piezoelectric layer 210.

The device may then be finished as shown in FIGS. 5I-5K. Specifically, avent opening may be formed through the elastic layer 204 to thesacrificial layer 202, as shown in FIG. 5I. Then a protective layer 218,e.g., an oxide may be formed over the front side of the device and theopening 207 may be formed through the substrate, e.g., by etching thesubstrate from the backside, stopping on the sacrificial layer 202, asshown in FIG. 5J. Any suitable isotropic or anisotropic etch process forthe substrate material may be used, for example deep reactive ionetching (DRIE) when substrate 201 is composed of silicon. The protectivelayer prevents damage to the structures formed on the membrane duringthe etch process. The sacrificial layer 202 and the protective layer 218may then be removed, e.g. by etching in hydrofluoric acid (HF) or vaporphase hydrofluoric acid (VHF) as shown in FIG. 5K.

Although certain specific examples of pMUT device in accordance withaspects of the present disclosure are shown in the drawings anddescribed hereinabove, the present disclosure is not limited to suchimplementations. For example, in some implementations the lowerelectrode layer 208 may be omitted if the membrane 204 is made of asufficiently electrically conductive material and the device is intendedto operate with the lower electrode grounded to the substrate 201. Insuch implementations, the membrane itself may act as the lowerelectrode.

The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.” Any element in aclaim that does not explicitly state “means for” performing a specifiedfunction, is not to be interpreted as a “means” or “step” clause asspecified in 35 USC § 112, ¶6. In particular, the use of “step of” inthe claims herein is not intended to invoke the provisions of 35 USC §112, ¶6.

What is claimed is:
 1. A piezoelectric micromachined ultrasonictransducer (PMUT) device, comprising: a substrate having an open cavity;a membrane attached to the substrate such that a portion of the membraneoverlies the open cavity; and an actuating structure on a surface of themembrane, the actuating structure including a piezoelectric layersandwiched between the membrane and an upper electrode layer, whereinthe actuating structure is patterned so that portions of the actuatingstructure are selectively removed from portions of the membrane to forma central portion proximate a center of the open cavity and three ormore rib portions projecting radially outward from the central portion,wherein the membrane is attached to the substrate at one or more anchorportions of the membrane proximate the perimeter of the open cavity, andone or more of the three or more ribs is patterned such that it ismechanically coupled to the central portion but electrically isolatedfrom the central portion.
 2. The device of claim 1, wherein theactuating structure is encapsulated by a passivation layer.
 3. Thedevice of claim 2, wherein the passivation layer is patterned such thatit is substantially removed from the portions of the membrane notcovered by the actuating structure.
 4. The device of claim 1, whereinthe three or more ribs include four or more rib portions.
 5. The deviceof claim 1, wherein the three or more ribs include six or more ribportions.
 6. The device of claim 1, wherein the three or more ribsinclude eight or more rib portions.
 7. The device of claim 1, whereinthe three or more ribs include one or more tapered rib portions.
 8. Thedevice of claim 1, wherein the three or more ribs include one or moretapered rib portions that are wider proximate the perimeter of themembrane than at the central portion.
 9. The device of claim 1, whereinthe membrane is perforated with one or more holes that pass through theactuating structure and the membrane.
 10. The device of claim 1, whereinthe actuating structure includes a lower electrode layer sandwichedbetween the piezoelectric layer and the membrane.
 11. The device ofclaim 1, wherein the perimeter of the open cavity is circular in shape.12. The device of claim 1, wherein the perimeter of the open cavity issquare in shape.
 13. The device of claim 1, wherein a shape of themembrane is a polygonal shape.
 14. The device of claim 1, furthercomprising an opening formed through the central portion of theactuating structure and through an underlying portion of the membrane tothe open cavity.
 15. The device of claim 1, wherein a ratio of a radiusof the central portion to a radius of the perimeter is between 0.4 and0.8.
 16. The device of claim 1, wherein a ratio of a radius of thecentral portion to a radius of the perimeter is about 0.6.
 17. A methodfor fabricating a piezoelectric micromachined ultrasonic transducer(PMUT) device, comprising: forming a membrane attached to a substrate;forming an actuating structure on a surface of the membrane, theactuating structure including a piezoelectric layer sandwiched betweenthe membrane and an upper electrode layer; patterning the actuatingstructure to selectively remove portions of the actuating structure fromportions of the membrane to form a central portion proximate a center ofan open cavity and three or more rib portions projecting radiallyoutward from the central portion; and forming an opening through aportion of the substrate underlying the membrane and the actuatingstructure such that the opening forms the open cavity, wherein themembrane is attached to the substrate at one or more anchor portions ofthe membrane proximate the perimeter of the open cavity, and one or moreof the three or more ribs is patterned such that it is mechanicallycoupled to the central portion but electrically isolated from thecentral portion.